[lostcolony]

If you're doing SCRUM, hopefully, when planning, you're building in slack for the devs already. Your sprint commitments should always be conservative. If something ends up taking more time than you planned, then, it'll just take up some of that slack.

With that slack, then, when things go right, you have time for dev priority tasks. You hit your sprint commitments two days before the sprint ends? Great! Time to work on some of those long term investments.

You can also scope stories based on dev priorities. You want to add a feature to part of the code that is in need of refactoring? Scope the story to include that refactoring.

Etc.

If using Kanban or something, it's a bit harder; you can easily take time for dev tasks (since you're not making commitments), but finding the right balance of taking them on but still getting feature work done is harder.